Most serial killers live a seemingly normal life when not engaging in murder. They can also be called serial killers if they use the same method each time. In addition, most (if not all) serial killers are highly intelligent, strategical, slippery, and very good liars and manipulators, as well as charming and seductive. Therefore, they are very difficult to catch, even for police.
In many cases, the best way to catch a serial killer is with the help from another serial killer, since they are the only people who know how another serial killer operates.